The present invention relates to photodetectors. More specifically, the present invention relates to avalanche photodiodes.
Owing to the known interaction between photons and electrons, advances have been made in the field of photodetectors in recent years, particularly in those photodetectors that utilize semiconductor materials. One type of semiconductor-based photodetector known as an avalanche photodiode includes a number of semiconductive materials that serve different purposes such as absorption and multiplication.
The avalanche photodiode structure provides a large gain through the action of excited charge carriers that produce large numbers of electron-hole pairs in the multiplication layer. In order to prevent tunneling in the absorption layer, the electric field is regulated within the avalanche photodiode itself, such that the electric field in the multiplication layer is significantly higher than that in the absorption layer.
A particular type of avalanche photodiode know as a mesa avalanche photodiode exposes high field p-n junctions and large numbers of exposed surface and interface states that make it difficult to passivate using a layer of insulating material. Therefore, conventional InP/InGaAs avalanche photodiodes use diffused structures which bury the p-n junction. However, these InP avalanche photodiodes require extremely accurate diffusion control of both the depth and the doping density of the p-type semiconductor regions as well as accurate control of the n-doped region into which this diffusion occurs. This critical doping control is essential, since the diffusion controls the placement of the p-n junction, the magnitude of the electric field in the multiplication region, the length of the avalanche region, as well as the total charge in the charge control layer which determines the values of the electric fields in both the high field InP avalanche region, which must be large enough to produce multiplication, as well as the low field InGaAs absorbing region, which must be small enough to avoid tunneling. In addition, accurately placed diffused or implanted guards rings are used in this type of arrangement, to avoid avalanche breakdown at the edges of the diffused p-n junction. This combination of guard rings and critically controlled diffusions increases the capacitance, lowers the bandwidth, and reduces the yield, thus increasing the cost of these APDs.
For ultrahigh speed performance detectors, InAlAs can be used as the avalanche layer rather than InP, since the higher bandgap reduces tunneling and thus allows thinner avalanche regions to be used leading to higher speeds and higher performance receivers. However, a diffused structure is even more difficult to achieve in InAlAs since the larger electron avalanche coefficient (relative to the holes) makes it desirable to multiply the electrons rather than the holes as in standard InP based APDS. Moreover, simply reversing the standard p-doped diffused structure is not sufficient, since n-dopants do not diffuse fast enough.